A vacuum lifting device is often used as a fixation tool e.g. when palletizing goods. The vacuum lifting device typically comprises an array of individual suction cups connected to a common vacuum pump so that when the lifting device is placed on an item and vacuum is established inside the suction cups, by means of the vacuum pump, the lifting device will be able to lift the item and move it around until it reaches its destination. Here the item is released by removing the vacuum inside the cups. If the item is irregularly shaped or for other reasons is difficult to handle, it is known to provide each suction cups with a flow blocking valve. A flow blocking valve ensures that that the entire lifting device does not release the load if a single (or a few) suction cups are not completely covered by the item, when the vacuum is established or if a suction cup loses its grip during lifting or displacement of the item.
A flow blocking valve usually comprises a spring loaded float inside the valve housing so that when the flow through the valve exceeds a certain level the floater is moved by the flow towards a seat and when the floater is sucked against the seat, the flow through the valve is blocked. Thus, the flow blocking valve seals the air passage from the suction cup to which it is attached, and thereby prevents a vacuum loss throughout the rest of the vacuum lifting device.
However, if the surface of the item is porous or otherwise “difficult”—i.e. it folds or changes shape when vacuum is locally applied—it is desired that the suction cups, which are not in contact with the item, are closed while the flow through the remaining cups is maintained—even if the connection between the cups and the item leaks. Thus, from WO 2008/019691 A1 it is known to provide the vacuum lifting device with arresting means comprising pins arranged to protrude down into the valve housing, coaxial with the direction of motion of the floater, inside the housing, to prevent the floater from reaching the seat and thus ensuring that the floater does not block the flow even if the vacuum pump establishes a more or less constant flow through the valve.
However, this solution is expensive, mechanically complex and unreliable.
The invention therefore seeks to provide for a simple and reliable flow blocking valve and a simple and reliable technique for lifting items by means of vacuum.